Television camera tube



y 1955 H. BRUINING ET AL TELEVISION CAMERA TUBE Filed Nov. '7, 1951 INVENTORS Hendri kus Johannes Lemmens H010 Bruining By Agent United States PatentO TELEVISION CAMERA TUBE Hajo Bruining and Hendrikus Johannes Lemmens, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application November 7, 1951, Serial No. 255,242

Claims priority, application Netherlands November 13, 1950 2 Claims. c1. 31365) This invention relates to television camera tubes.

In television camera tubes in which the image to be transmitted is projected onto a photo-electric cathode of a so-called image amplifier by which it is electron-optically projected onto the image electrode, the presence of the photo-electric cathode gives rise to difficulties. This photo-electric cathode is obtained by precipitating an alkaline metal on a conductive layer of a suitable metal, for example, silver or antimony. Alkaline metal, as a rule, caesium is introduced into the tube in the form of vapour and after the required amount of caesium to obtain the desired sensitiveness has been precipitated on the photo-cathode, the excess vapour is pumped out of the tube. The excess caesium which cannot thus be removed from the tube may be rendered innocuous by a gas binder which, as a rule, is provided as a deposit on the tube wall.

This measure, in general, mitigates sufiiciently the disadvantageous effect due to the alkaline vapour making contact with the photo-cathode surface and other parts of the tube. Deposited caesium may give rise to secondary emission phenomena and reduce the surface insulation of insulating parts.

The said measures, however, are not suflicient for television transmission tubes, comprising an electrode system for producing a directional electron beam which is directed towards the image screen on the same side as the photo-cathode and arranged in a side-branch of the tube. It has been found that in these tubes the insulation of the parts supporting the electrodes deteriorates gradually and the life of the tubes is seriously reduced.

The object of the invention is to provide an improved television tube.

According to the invention a television camera tube for producing electron-optical image magnification by photoemission and electrical acceleration of electrons, comprising a scanning electrode system provided on the same side of the image electrode as the electron-optical projection means and arranged in a side-branch of the tube, is characterized in that a metal wall coating is applied as a conductive layer to the inner wall of the side-branch in order to avoid unequal voltage distribution thereon and at its end nearer the electrode system is of a metal which binds caesium vapour. Use is preferably made of gold which may readily be applied in the form of a thin coating.

The caesium binding coating should not extend within the range of action of the magnetic deflection fields since due to the absorption of caesium, the electrical resistance of the coating and hence the damping which is produced by such a metal layer in an alternating magnetic field varies.

Particular substances capable of binding caesium vapour at room temperature, for example, aquadag and graphite, cannot be used in this case, since with a small increase in temperature, which may readily occur owing to their proximity to the cathode, they give off some absorbed vapour. Even at high temperatures gold and silver retain caesium. Gold absorbs caesium less rapidly than silver. Since the area of arrangement of the binder is 2,713,129 Patented July 12, 1955 located remote from the photo-cathode and communicates with the space into which the vapour is introduced through a comparatively narrow port, so that a comparatively small supply of caesium finds its way to this area during manufacture of the photo-cathode use could be made of silver without the risk of a failure of the binding activity owing to premature saturation. V

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying diagrammatic drawing, given by way of example, showing one embodiment of a television camera tube according thereto.

Referring now to the figure, a camera tube 1 comprises a glass envelope of substantially circular section in the plane at right angles to the figure. A narrow cylindrical part 2 of the tube merges into a wider part 3 which is closed by a substantially spherical bottom 4. An image electrode 7 is secured by way of a support 5 to the bottom 4, to which support 5 is also secured a current supply wire 6. The construction of such an image electrode is known and will not be further described.

The narrow cylindrical part 2 of the tube 1 is closed by a fiat plate 8. The plate 8 is coated with a thin, transparent, conductive layer 9, for example, of iridium. A thin metal layer 10, for example, of silver or antimony is applied by evaporation to the layer 9. The layer 10 is activated by introducing oxygen and caesium vapour into the tube. When the required degree of photosensitiveness is reached, the excess caesium vapour is exhausted in known manner. In order to mitigate the effect of residual caesium, a deposit 11 of one of the known caesium-binding substances may be applied to the wall of the tube.

Electrons emitted by the photo-cathode may be concentrated on the image electrode 7 by a magnetic field produced by a coil 12 and produce a potential image which corresponds with the electron image emitted by the photo-cathode.

For scanning the image electrode 7, an electrode system 13 arranged on the same side of the electrode 7 as the photo-cathode and directed towards the image electrode is housed in a side-branch 14 secured to the tube wall and projecting sideways therefrom. The electron beam produced by this system is focussed by means of a magnetic field produced by a coil 15, the required deflection of the electron beam for scanning the image electrode 7 being produced with the use of a coil system 16. The inner surface of the side-branch 14 is coated with a conductive layer 17 in order to neutralize wall charges. This wall coating, which may, for example, be of aluminium, may extend into the interior of the tube and thus operate at the same time as a collecting electrode for secondary electrons produced when electrons strike the image electrode 7. At its end near the electrode system 13, the conductive coating 17 is provided with a rim 18 which is a layer of a metal, preferably gold, suitable for binding caesium vapour. In this manner, inadmissible increase in electrical conductivity of those surfaces which are used for the insulated arrangement of the electrodes of the electrode system 13, is counteracted.

What we claim is:

1. A television camera tube for producing electronoptical image magnification comprising a vitreous envelope, including a side-branch, a caesium-activated photo-emissive layer disposed on the inner surface of said envelope at one end thereof, an image electrode disposed within said envelope at the other end thereof, electronoptical means for projecting an electron image produced by said photo-emissive layer onto said image electrode, a scanning electrode system disposed on the same side of the image electrode as the photoemissive layer and within the side-branch of the envelope and positioned to scan said image electrode with an electron beam, and a metal conductive coating on the inner surface of the wall of said side-branch of the envelope containing said scanning electrode system to prevent unequal potential distributionsthereon, the portion of said conductive coating adjacent said electrode system being constituted by a metal capable of binding caesium vapour. 7

2. A television camera tube as claimed in claim 1, in which the portion of the conductive layer for binding caesium vapour is constituted by gold.

References Cited in the file of this patent UNITED STATES PATENTS A. M. Glover Feb. 17, 1942 

